The present invention relates to a method for the deposition of a ternary, quaternary or similar composition, especially a phosphor, in which components of the composition are located on different sources. In particular, the compositions are thioaluminates, thiogallates or thioindates of Group IIA and Group IIB elements, and the sulphides that form such compounds are located on the different sources. A preferred method of deposition is electron beam vaporization. In particular aspects, the present invention relates to a method of improving the luminance and emission spectrum of phosphor materials, especially those used for full colour ac electroluminescent displays employing thick film dielectric layers with a high dielectric constant.
Thin film electroluminescent (TFEL) displays are known and are typically fabricated on glass substrates. However, electroluminescent displays with thin film phosphors employing thick film dielectric layers fabricated on ceramic substrates, as exemplified by U.S. Pat. No. 5,432,015, provide greater luminance and superior reliability.
Thick film dielectric structures provide superior resistance to dielectric breakdown, as well as a reduced operating voltage. When deposited on a ceramic substrate, the thick film dielectric structure will withstand higher processing temperatures than TFEL devices on glass substrates. The increased tolerance to higher temperatures facilitates annealing of the phosphor films at higher temperatures, to improve luminosity. However, even with the enhanced luminosity that is obtained, electroluminescent displays employing a thick film dielectric layer have not achieved the phosphor luminance and colour coordinates needed to be fully competitive with cathode ray tube (CRT) displays. Moreover, recent trends in CRT specifications are to higher luminance and higher colour temperature. Some improvement has been realized by increasing the operating voltage of electroluminescent displays, but this increases the power consumption of the displays, decreases the reliability and increases the cost of operating electronics for the displays.
A high luminosity full colour electroluminescent display requires the use of red, green and blue sub-pixels. Optical filters are needed to achieve the required colour coordinates for each sub-pixel. Consequently, the thin film phosphor materials used for each sub-pixel must be patterned so that there is minimal attenuation of the emission spectrum for each colour of pixel by the optical filters. For relatively low-resolution displays, the required patterning can be achieved by depositing the phosphor materials through a shadow mask. For displays with high resolution, however, the shadow mask technique does not provide adequate accuracy, and photolithographic methods must be employed. Photolithographic techniques require the deposition of photoresist films and the etching or lift-off of portions of the phosphor film to provide the required pattern.
Deposition and removal of photoresist films and etching or lift-off of phosphor films typically require the use of solvent solutions that contain water or other protic solvents. Some phosphor materials, for example strontium sulphide are susceptible to hydrolysis, and water and protic solvents may degrade the properties of the phosphor materials.
The deficiencies in phosphor materials are most severe with the phosphors used for blue sub-pixels, and may be compensated for to some extent by increasing the area of the blue sub-pixels relative to the area of the red and green sub-pixels. However, such a design modification demands increased performance from the phosphor materials used for the red and green phosphor materials, and requires the use of higher display operating voltages. The higher operating voltages increase the power consumption of the display, decrease the reliability and increase the cost of operating the electronics of the display.
Traditionally, cerium-activated strontium sulphide for blue and manganese-activated zinc sulphide for red and green have been the phosphor materials of choice for full colour electroluminescent displays. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary colour coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per watt of input power and the cerium activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per watt, relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with that for the zinc sulphide-based phosphor spanning the colour spectrum from green to red and that for the strontium sulphide-based material spanning the range from blue to green. This necessitates the use of the optical filters to obtain acceptable colour coordinates. The spectral emission of the cerium activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternate blue phosphor materials that have narrower emission spectra tuned to provide the colour coordinates required for blue sub-pixel have been evaluated. These include cerium activated alkaline earth thiogallate compounds, which give good blue colour coordinates, but have relatively poor luminosity and stability. Since the host materials are ternary compounds, it is relatively difficult to control the stoichiometry of the phosphor films. Europium-activated barium thioaluminate provides excellent blue colour coordinates and higher luminance, but it too is a ternary compound whose stoichiometry is difficult to control.
Vacuum deposition of phosphor films comprising europium-activated barium thioaluminate from a single source pellet using sputtering or electron beam evaporation has not yielded films with high luminosity. Improved luminance of barium thioaluminate phosphors has been achieved by using a hopping electron beam deposition technique to deposit films from two source pellets. The stoichiometry of the deposited film is controlled by controlling the relative dwell time of the electron beam impinging on each of the two source materials. However, this technique is not readily scalable to facilitate commercial production of large area displays and the process cannot be controlled to compensate for changes in the evaporation rates from the two sources as the deposition proceeds and the source pellets are depleted.
A method for the deposition of zinc sulphide thin films on transparent substrates was disclosed in JP 63-259067 of Shiro Kobayashi et al.
Improvements in methods of deposition of compositions, especially phosphors, to improve the luminance and emission spectrum of phosphor materials for electroluminescent displays employing thick film dielectric layers would be useful.
A method for the deposition of compositions e.g. phosphors, especially ternary and other chemically complex phosphors, has now been found.
Accordingly, one aspect of the present invention provides a method for the deposition of a thin film of a pre-determined composition onto a substrate, said composition comprising a ternary, quaternary or higher compound, comprising the steps of:
(i) placing a pellet of at least one sulphide on a first source and placing a pellet of at least one sulphide on a second source, the sulphides on the first and second sources being different, said sulphides being the components of said composition, at least one of the pellets on the first and second sources additionally containing dopant for the composition;
(ii) effecting vapour deposition of said composition on said substrate by simultaneously vaporizing the pellets on the first and second sources with separate electron beams; and
(iii) monitoring the rate of vaporizing of sulphide from the first source with a first coating rate monitor and monitoring the rate of vaporizing of sulphide from the second source with a second coating rate monitor, said first coating rate monitor being shielded from deposition of sulphide from the second source and said second coating rate monitor being shielded from deposition of sulphide from the first source.
In a preferred embodiment of the present invention, said first and second coating rate monitors are at a distance from the respective sources that is substantially the same as the distance of the substrate from said sources.
In another embodiment, the temperature of said first and second sources is controlled. Preferably, the temperature of each of the first and second coating rate monitors is monitored and controlled.
In further embodiments, the composition is a thin film phosphor or a dielectric thin film.
In further preferred embodiments, said composition is selected from the group consisting of ternary, quaternary and higher compositions of at least one cation from Groups IIA and IIB of the Periodic Table, especially thioaluminates, thiogallates and thioindates of at least one cation from Groups IIA and IIB of the Periodic Table.
In other embodiments, sulphide is located at a third source, said third source having a coating rate monitor that is screened from the first and second sources, said rate of coating from the third source being monitored and controlled. In particular, the third coating monitor is at a distance from the third source that is substantially the same as the distance of the substrate from said source.
In further preferred embodiments, said substrate is opaque in the visible and infrared regions of the electromagnetic spectrum.
In another aspect, the present invention provides a method for the deposition of a thin film of a pre-determined composition onto a substrate, said composition comprising a ternary, quaternary or higher composition, comprising the steps of:
(i) placing a first deposit at a first source of a vapour deposition apparatus and placing a second deposit at a second source of the vapour deposition apparatus, said first and second deposits being different, components of said first and second deposits in combination forming said pre-determined composition;
(ii) determining temporal variation of deposition of said components onto said substrate from said first and second sources; and
(iii) using said temporal variation for controlling said first and second sources so as to obtain homogeneous temporal deposition of said composition on the substrate by simultaneous vapour deposition from said sources.
In preferred embodiments of the method, said temporal variation is obtained by monitoring the rate of vaporizing from the first source with a first coating rate monitor and monitoring the rate of vaporizing from the second source with a second coating rate monitor, said first coating rate monitor being shielded from deposition from the second source and said second coating rate monitor being shielded from deposition from the first source.
In another embodiment, said monitoring is used as the determining of temporal deposition of step (ii). In particular, said monitoring of step (ii) may be used in step (iii).